At present, in a fuel injection control system for a multi-cylinder diesel engine, rotational speed sensors are usually provided both on a crankshaft and a camshaft, so as to detect the rotational speed of the diesel engine and judge the cylinders. In a single-cylinder engine, however, since the space for the camshaft is limited, the rotational speed sensor is only provided on the crankshaft. However, the injection timing cannot be directly determined by the rotational speed of the crankshaft speed alone, but a series of calculations and comparisons of the rotational speed of the crankshaft speed is also required to determine the injection timing of the diesel engine, thereby prolonging the response time and reducing the control accuracy.